What is viscosity? How does viscosity depend on intermolecular forces? What other factors affect viscosity?

When we talk about viscosity, we're essentially discussing how a fluid behaves when it's in motion. It's a bit like the 'thickness' you might associate with honey compared to water. But scientifically, the root of this 'thickness' is all about the forces that act between the molecules in a fluid, known as intermolecular forces.

Think of it this way. If the intermolecular forces are the 'handshakes' holding molecules together, then the stronger the handshake, the harder it is for the molecules to move past one another, hence, a higher viscosity. At a microscopic level, these forces vary depending on what the fluid is made of. For instance, the molecules in oil have different types of handshakes compared to water molecules, which is why oil is more viscous.

Some common types of these forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. The type and strength of these handshakes can drastically affect how easily a fluid flows.

The concept of fluid resistance is directly tied to viscosity. Think of resistance as a kind of internal friction within the fluid that resists its flow. When a fluid has high viscosity, you can imagine that it's filled with tons of tiny obstacles that each molecule has to get past in order to move; this is the fluid's resistance.

It's the same reason why swimming in syrup would be much harder than swimming in water—the syrup presents more resistance to your movements because its viscosity is higher. The field of fluid dynamics studies this aspect intensely to solve practical problems, like predicting how a car's design will affect air flow around it, or how oil flows through pipelines.

The relationship between temperature and viscosity is quite fascinating. As temperature increases, the viscosity of a fluid typically decreases. This happens because higher temperatures give the molecules more energy; they start to move faster and can more easily break free from the 'handshakes' we talked about earlier.

Consider heated honey; it pours more easily than cold honey. That’s because the heat causes the honey molecules to move more vigorously, reducing the effect of their intermolecular forces. In technical terms, we say that the kinetic energy of the molecules increases with temperature, thereby reducing viscosity.

Now, the impact of pressure on viscosity is less straightforward than temperature. Generally, increasing pressure compacts the molecules more closely together, which you’d think would increase the viscosity. However, it doesn't always work like that.

For many fluids, especially liquids that are already pretty incompressible, a change in pressure doesn't significantly alter their viscosity. But for gases, which are more compressible, cranking up the pressure usually also cranks up their viscosity. This behavior can be critical in engineering applications where gases are compressed and transported through pipelines, like natural gas in energy industries.